Electrochemical devices, including proton exchange membrane fuel cells, sensors, electrolyzers, chlor-alkali separation membranes, and the like, have been constructed from membrane electrode assemblies (MEAs). Such MEAs comprise at least one electrode portion, which include a catalytic electrode material such as Pt in contact with an ion conductive membrane. Ion conductive membranes (ICMs) are used in electrochemical cells as solid electrolytes. In a typical electrochemical cell, an ICM is in contact with a cathode and an anode, and transports ions that are formed at the anode to the cathode, allowing current to flow in an external circuit connecting the electrodes. The central component of an electrochemical cell, such as a fuel cell, sensor, electrolyzer, or electrochemical reactor, is the 3-layer membrane electrode assembly, or MEA. It consists, in the most general sense, of two catalyzed electrodes between which is sandwiched an ion conducting electrolyte, preferably a solid polymer electrolyte for the applications of this invention. This 3-layer MEA is in turn sandwiched between two porous, electrically conducting elements called electrode backing layers (EBLs), to form a 5-layer MEA.
MEAs can be used in sensors and hydrogen/oxygen fuel cells. A typical 5-layer MEA for use in a hydrogen/oxygen fuel cell might comprise a first EBL, a first Pt electrode portion, an ICM containing a proton-exchange electrolyte, a second Pt electrode portion, and a second EBL. Such a five-layer MEA can be used to generate electricity by oxidization of hydrogen gas, as illustrated in the following reactions: ##STR1##
In a typical hydrogen/oxygen fuel cell, the ions to be conducted by the membrane are protons. Importantly, ICMs do not conduct electrons/electricity, since this would render the fuel cell useless, and they must be essentially impermeable to fuel gasses, such as hydrogen and oxygen. Any leakage of the gasses employed in the reaction across the MEA results in waste of the reactants and inefficiency of the cell. For that reason, the ion exchange membrane must have low or no permeability to the gasses employed in the reaction.
ICMs also find use in chlor-alkali cells wherein brine mixtures are separated to form chlorine gas and sodium hydroxide. The membrane selectively transports sodium ions while rejecting chloride ions. ICMs also can be useful for applications such as diffusion dialysis, electrodialysis, and pervaporization and vapor permeation separations. While most ICMs transport cations or protons, it is known in the art that membranes can be prepared that are transportive to anions, such as OH.sup.-.
The ICM typically comprises a polymeric electrolyte material, which may constitute its own structural support or may be contained in a porous structural membrane. Cation- or proton-transporting polymeric electrolyte materials may be salts of polymers containing anionic groups and nearby fluorocarbon groups.
Fuel cell MEAs have been constructed using catalyst electrodes in the form of applied dispersions of either Pt fines or carbon supported Pt catalysts. The predominant catalyst form used for polymer electrolyte membranes is Pt or Pt alloys coated onto larger carbon particles by wet chemical methods, such as the reduction of chloroplatinic acid. This conventional form of catalyst is dispersed with ionomeric binders, solvents and often polytetrafluoroethylene (PTFE) particles, to form an ink, paste or dispersion that is applied to either the membrane, or the electrode backing material. In addition to mechanical support, it is generally believed in the art that carbon support particles provide necessary electrical conductivity within the electrode layer.
In another variation, a catalyst metal salt can be reduced in an organic solution of a solid polymer electrolyte to form a distribution of catalyst metal particles in the electrolyte, without a support particle, which can then be cast onto an electrode backing layer to form the catalyst electrode.
In a further variation, Pt fines can be mixed directly with a solution of solvents and polymer electrolyte and coated onto the electrode backing layer or membrane ICM. However, because of limitations on how small the fines can be made, this approach typically results in very high, and therefore expensive, loading of the catalyst.
Various other structures and means have been used to apply or otherwise bring a catalyst in contact with an electrolyte to form electrodes. These MEAs can include: (a) porous metal films or planar distributions of metal particles or carbon supported catalyst powders deposited on the surface of the ICM; (b) metal grids or meshes deposited on or imbedded in the ICM; or (c) catalytically active nanostructured composite elements embedded in the surface of the ICM.
The prior art teaches that an effective MEA design must maximize contact between the catalyst and the ionomer electrolyte in order to obtain higher efficiency and capacity to handle higher currents. It is reportedly crucial to maximize the three-phase interface between the catalyst, ionomer and the gaseous reactants which may permeate the ionomer. To that end, a primary objective of previous research has been to optimize catalyst utilization by maximizing the surface area of catalyst which is in contact with the ion exchange resin or ionomer, in order to effectively facilitate the exchange of protons between the catalyst surface site of the redox reactions and the ion conduction membrane. Catalyst not in direct complete contact with the ionomer has been termed "non-reacting" catalyst.
Nanostructured composite articles have been disclosed. See, for example, U.S. Pat. Nos. 4,812,352, 5,039,561, 5,176,786, 5,336,558, 5338,430, and 5,238,729. U.S. Pat. No. 5,338,430 discloses that nanostructured electrodes embedded in solid polymer electrolyte offer superior properties over conventional electrodes employing metal fines or carbon supported metal catalysts, particularly for sensors, including:
protection of the embedded electrode material, more efficient use of the electrode material, and enhanced catalytic activity.